Add more test cases and improve code quality

[cmHuang777]

• Added more test cases
• removed dead code in model class and its implementation

[codecov[bot]]

Codecov Report

Merging #219 (f953cd9) into master (c6594dd) will increase coverage by 2.86%. The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master     #219      +/-   ##
============================================
+ Coverage     72.20%   75.06%   +2.86%     
- Complexity      724      755      +31     
============================================
  Files           112      112              
  Lines          2547     2531      -16     
  Branches        274      271       -3     
============================================
+ Hits           1839     1900      +61     
+ Misses          601      538      -63     
+ Partials        107       93      -14     

Files	Coverage Δ
.../address/logic/commands/AddAppointmentCommand.java	100.00% <ø> (+27.45%)	:arrow_up:
src/main/java/seedu/address/model/AddressBook.java	78.12% <ø> (+6.48%)	:arrow_up:
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)
.../main/java/seedu/address/model/person/Patient.java	93.02% <100.00%> (+4.65%)	:arrow_up:
.../java/seedu/address/storage/JsonAdaptedDoctor.java	100.00% <100.00%> (ø)
...java/seedu/address/storage/JsonAdaptedPatient.java	100.00% <100.00%> (+6.25%)	:arrow_up:
.../java/seedu/address/storage/JsonAdaptedPerson.java	100.00% <ø> (+10.00%)	:arrow_up:
...rc/main/java/seedu/address/model/ModelManager.java	89.00% <75.00%> (+7.09%)	:arrow_up:

... and 11 files with indirect coverage changes

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